Tryptophan hydroxylase 1 with bound tryptophan

The catalytic domain of chicken tryptophan hydroxylase 1 with bound tryptophan

Tryptophan hydroxylase is an iron and tetrahydrobiopterin dependent monooxygenase which belongs to the enzyme family of aromatic amino acid hydroxylases. The structure presented here is of the catalytic domain of chicken tryptophan hydroxylase 1 with bound tryptophan substrate.

Tryptophan binding

The is bound in a binding pocket distinct from the tetrahydrobiopterin binding pocket. Hydrogen bonds between tryptophan and TPH are shown by dashed lines and hydrophobic interactions with the semi-circled residues. For residues that are not conserved in phenylalanine hydroxylase (PAH) and tyrosine hydroxylase (TH) the corresponding residues are shown.

The substrate specificity of TPH1 has been studied previously. These studies have shown that the substrate specificity i mainly controlled by ^[1] and ^{[2] [3]}. Tyr236 is also involved in as seen in the structure of human TPH1 (1mlw) and in the binding of tryptophan, as seen in this structure. Phe314 is a smaller amino acid than the corresponding tryptophan seen in TH and PAH. Phenylalanine instead of tryptophan at position 314 results in a larger binding cavity in TPH with room for the tryptophan substrate.

Structural changes upon tryptophan binding

When comparing the structure of chicken TPH1 with structure of the human TPH1 with bound (entry 1mlw), large structural changes are observed. These structural changes can be understood as the enzyme closing around the active site when tryptophan is bound. The structural changes are illustrated in the figure below showing the structural alignment of two TPH1 structures (3e2t and 1mlw). The positions of Leu130 and Ile366 are shown in the two structures. These two amino acids are positioned on the two loops that move toward each other when tryptophan binds. This movement reduces the distance between the two amino acids by 10 Å.

Crystal structures of phenylalanine hydroxylase have been solved with a substrate analogue and tetrahydrobiopterin (entry 1mmk). This structure shows the same compactness as the chicken TPH1 structure with an root mean square deviation of 0.94 Å. In comparison the chicken TPH1 and human TPH1 structure has an r.m.s.d. of 1.47 Å.

Iron coordination

The by 2 histidines, one glutamate and one imidazole from the solvent. This coordination is called the 2-histidine-1-glutamate facial triad iron coordination and is seen in many mononuclear non-heme iron(II)enzymes^[4]. In this structure Glu317 coordinates the iron in a partial bidentate manner. The more common (the resting state) for the 2-His-1-Glu iron coordination is seen in the structure of human TPH1 (1mlw).

Imidazole binding

The is bound to the iron and to the protein chain through two brigding water molecules. The first water molecule makes hydrogen bonds to Gly235 and to Leu237 while the other water molecule makes hydrogen bonds to His252 and Glu274. This binding is similar to the binding in structure of the catalytic domain of human TPH1 (1mlw).